A coating composition intended for use on the outside of cans as a base for printing labels with multiple colors of inks must meet several stringent requirements. One of the requirements is good adhesion to the substrate which may be aluminum, electrolytically tin-plated steel, tin-free steel, or other materials. Also, multicolor labels often require baking a number of times to cure each color printing before the next is applied. Coating compositions used as a base coat, generally white, to be subjected to such multiple bakes, must not exude any ingredients that would interfere with the inks. Moreover, the base coat must cure fast enough for economical high speed production. Furthermore, a rather high level of pigmentation is needed to provide white opacity for the printing base, and a high solids content is desirable to minimize solvents which are costly and can contribute to air pollution.
More or less flexibility is also needed in the cured coating composition. For three-piece cans, most of the can forming is done after applying the base coat and also the ink patterns. Thus, coatings for three-piece cans need to be quite flexible. The body of a three-piece can is generally formed from base coated and printed flat metal stock which is cut to size, rolled to form a cylinder, and a seam is made. The other two pieces in a three-piece can are the ends. One is applied before filling the can, the other after. Coatings for two-piece cans still require flexibility, but less so than those for three-piece cans. The body of a two-piece can is made by drawing and ironing a cup-shaped body from a flat sheet of metal. The outside of this formed body is the part to which the base coat of the invention is applied. A flat top is provided to be sealed onto the can after the can has been filled. In the sealing of this top, some flexibility of the base coat on the can body is desirable.
The ideal combination of adhesion to the substrate, minimal exudation upon multiple bakes, and opacity in thin flexible coatings that cure rapidly is not easy to produce in a high solids coatings. Adequate dispersion stability is also necessary for commercial success. Exterior can coatings differ from other finishes such as automotive and exterior architectural finishes in the ways described above and also in not needing similar durability to weather, ultraviolet light and physical abuse.
Prior exterior can finishes often used pigment dispersants and flow control agents which tended to exude and interfere with printed ink patterns on multiple bakes to cure multiple layers of ink. The interference would cause the ink and the overlying clear varnish to lift off. Adhesion to the substrate was also inadequate.